House Estermont is a vassal house from the Stormlands that holds fealty to House Baratheon. Their seat is Greenstone, a castle located on the island of Estermont in Shipbreaker Bay. Their sigil is a dark green sea turtle on a field of pale green.

In the A Song of Ice and Fire novels, Lady Cassana had many siblings including two brothers. The elder, Lord Eldon Estermont, is about 70 years old and head of House Estermont, and the younger, Ser Lomas Estermont, has a son, Ser Andrew. Ser Andrew Estermont had previously been Lord Stannis Baratheon’s squire for numerous years.

The first time Queen Cersei Lannister became aware of her husband's infidelity was when they repaid a visit to the Estermonts. Queen Cersei was miserable the entire fortnight they spent with the Estermonts while King Robert enjoyed hunting, hawking, drinking, and play fighting with his maternal uncles and male cousins. He also took to reconnecting and comforting a female cousin whose father and husband died in the siege of Storm's End and eventually spent the night with her. That same night Ser Jaime Lannister offers to kill King Robert, but Queen Cersei says she wants him "horned" instead. She likes to think that was the night when her son Joffrey was conceived.

Following the Battle of Blackwater Bay, Lord Eldon bends the knee to King Joffrey, though some members of House Estermont remain loyal to Lord Stannis.